I do know that I drive torque, while listening to my friends prattle on about horsepower.
Use a multimeter. That way you can get a precise reading. The way you are doing it is correct but with a multimeter set it to read amps, then connect one end to the negative terminal on the battery and the other on the ground cable end.25-30 miliamps is considered a normal draw on your battery. More than that and you have a parasitic draw. I had one once. Turns out it was the way I had my radio wired.